Conversion of agricultural land from conventional tillage (CT) to no-till (NT) is widely considered to be one of the key strategies for terrestrial C sequestration. However, estimates of annual C sequestration rates published in the literature vary widely due to differences in cropping systems, cropping practices, and methodologies used for estimating changes in soil C over time. In this paper, we address four critical issues: (1) measurement errors (sampling, sample processing, measurement of C, bulk density), (2) wrong calculation of soil C and N stocks: constant soil volume vs. constant soil dry mass, (3) biased estimates of annual C sequestration rates: relative difference in SOC (e.g., NT relative to CT) vs. absolute changes over time, and (4) lack of landscape-level studies. Using data sets from several long-term studies, we show that these errors may often lead to significant overestimation of true soil C sequestration rates under no-till cultivation.